1. Field of the Invention
The present invention relates to a magnetic field detector and a medical implant having a combined telemetry and magnetic field detector unit.
2. Description of the Prior Art
In a medical implant, e.g. a pacemaker, a magnetic field detector is used for non-invasive activation of different implant functions with the aid of a permanent magnet brought close to the implant on the outside of the patient's body. Some of the functions in a pacemaker which can be activated with a magnet are, for example: having the pacemaker disable the demand function so the pacemaker adapts its operation to the capacity of the battery, having the pacemaker operate according to a special, temporary stimulation mode (such as in the presence of tachycardia), and in conjunction with pacemaker programming.
A magnetic field detector intended for a medical implant should be small (the latest generation of pacemakers only weighs about 14 grams), insensitive to bumps and should suit the assembly technique used for other components in the pacemaker.
The detection of magnetic fields outside the implant art in a number of different ways is generally known, e.g. with the aid of reed switches and Hall generators utilizing the Hall effect. The disadvantage of a Hall generator in the context of use in a medical implant is its relatively high energy consumption.
An electronic sensor for detecting static magnetic fields is described in U.S. Pat. No. 4,611,127. The sensor has a conventional resonant circuit with a coil used to sense the presence of a magnetic field whose strength exceeds a defined value. The resonant frequency of the resonant circuit varies according to the strength of the magnetic field. The resonant circuit is activated periodically, and the number of zero crossings of the output signal from the resonant circuit is analyzed in a sensing window of a defined duration. If a given number of zero crossings occurs in the sensing window, this means that the strength of the magnetic field exceeds the defined value.
A magnetic field detector of the conventional kind in the implant art consists of a reed switch. Reed switches, however, are relatively insensitive and rather expensive components which also occupy a relatively large volume in the implant.
To eliminate the need for a reed switch, therefore, utilization of the implant's telemetry unit for detecting the presence of a magnetic field, in addition to performing telemetry functions, has been proposed in recent years.
U.S. Pat. No. 4,541,431 discloses one such proposal with a combined telemetry and magnetic field detector in which magnetic field sensing is performed in the same way as described in U.S. Pat. No. 4,611,127 cited above. The combined unit has a conventional resonant circuit formed by a coil used in telemetry for transmitting and receiving data. The resonant circuit is also used for sensing, in the above-described manner, the presence of a magnetic field whose strength exceeds a defined value.